c Indian Academy of Sciences.
Bull. Mater. Sci., Vol. 39, No. 3, June 2016, pp. 803–809. DOI 10.1007/s12034-016-1190-2
Oxygen evolution reaction of Ti/IrO2 –SnO2 electrode: a study by cyclic
voltammetry, Tafel lines, EIS and SEM
GAYANI CHATHURIKA PATHIRAJA1,4 , NADEESHANI NANAYAKKARA1,3,4,∗ and
ATHULA WIJAYASINGHE2
1 Environmental Engineering/Electrochemistry Research Group, Institute of Fundamental Studies,
Kandy 20000, Sri Lanka
2 Nanotechnology/Physics of Materials Research Group, Institute of Fundamental Studies,
Kandy 20000, Sri Lanka
3 Department of Civil Engineering, Faculty of Engineering, University of Peradeniya, Peradeniya 20400, Sri Lanka
4 Post Graduate Institute of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka
MS received 15 May 2015; accepted 3 December 2015
Abstract. The electrochemical characteristics towards oxygen evolution reaction of thermally prepared
Ti/IrO2 –SnO2 electrodes were examined. Two electrodes prepared with two different IrO2 compositions were analysed in Na2 SO4 electrolyte. Cyclic voltammetry, steady state polarization curves, impedance spectroscopy and open
circuit potential were performed to investigate the performance and stability of these electrocatalysts. It has found
that the surface electrochemistry of Ti/IrO2 –SnO2 anodes are governed by the Ir(III)/Ir(IV) couple. The impedance
spectroscopy investigation permitted to propose an equivalent circuit to describe the modifications occurred in different potentials during oxygen evolution reaction. The same equivalent circuit was found to describe both electrodes.
Moreover, the double layer capacitance and microstructure analysis reflected that the inner surface makes a large
contribution to the electrochemically active surface area of Ti/IrO2 –SnO2 anodes. It was found that the stability and
the electrocatalytic activity mainly depend on changes in the IrO2 composition of the electrode and its morphology
during oxygen evolution reaction.
Keywords.
1.
Oxygen evolution reaction; impedance spectroscopy; electrocatalysts; microstructure.
Introduction
Oxygen evolution reaction (OER) on dimensionally stable
anodes (DSAs) has been a subject of much interest in the
electrochemical industry. An interest in Ti/IrO2 type DSAs
for oxygen evolution has been attracted in the last decades
also [1,2]. The advantage of these electrodes as one of the
most widely employed OER electrocatalyst lie in their high
catalytic activity, stability, extremely high durability, high
electrical conductivity and corrosion resistance [1–4]. As an
‘active’ electrode (electrodes which have low over-potentials
for the OER), IrO2 is believed to be less sensitive to poisoning due to simultaneous oxygen evolution [5]. The electrocatalytic activity of IrO2 electrode can be further improved
by controlling the electrode preparation parameters and the
appropriate selection of the oxide components as secondary
metals [6,7]. Various kinds of these anodes can be developed by mixing with ‘non-active’ electrode materials such
as TiO2 , ZrO2 , Nb2 O5 , SnO2 , Ta2 O5 and they have been
extensively studied as oxygen-evolving anodes [4,8–12].
Electrochemical impedance spectroscopy (EIS) is used as
a powerful technique in characterization of electrochemical
properties of metallic oxide electrodes. Equivalent circuits
∗ Author
for correspondence ([email protected])
were used to describe the influence of each component of
the oxide layer on the impedance spectrum. Typical electric
circuits are modelled to the oxide electrode elements that
describe the ohmic resistance associated with solution and
the oxide film, as well as the charge transfer resistance of
the processes that occur at the oxide/solution interface [2,13–
15]. Moreover, the superior electrocatalytic properties of an
anode material towards oxygen evolution can be achieved
by high exchange current density and low Tafel slope. The
anode activity depends also on the electrochemically active
surface area. As a consequence, investigating novel electrode
materials with better electrocatalysis properties and lower
overpotential for OER is crucial [6].
Since iridium is expensive and scarce, in this research,
alternative electrodes mixed with less expensive electrocatalyst SnO2 were investigated. In our previous work, it has
been found that such electrodes provide better efficiencies
in degrading organics dissolved in water, and the electroactive surface area and stability are high [17]. The goal of
the present work is to find electrochemical properties of OER
that developed Ti/IrO2 –SnO2 electrode in Na2 SO4 solution.
So far, many researchers have been carrying out a large number of investigations on the works of IrO2 -based DSAs in
acidic and alkaline solutions for OER [2,13,15]. However,
little work is known about investigation for OER of IrO2
803
804
Gayani Chathurika Pathiraja et al
type electrodes in neutral solutions [16]. Moreover, better
performances in OER will allow the electrode to be used
as oxidizing organics in wastewater, even the wastewater
contains low amount of chloride ions.
To the best of our knowledge, no published study has
investigated the electrocatalytic properties of Ti/IrO2 –SnO2
electrode in Na2 SO4 solution. To investigate these changes,
the surface properties of IrO2 –SnO2 system with two different IrO2 compositions were analysed using the OER
as a model reaction in a neutral medium. The electrocatalytic properties of these anodes were investigated in terms
of cyclic voltammetry, open circuit potential, linear sweep
voltammetry and EIS.
2.
Experimental
2.1 Electrode preparation
Electrodes with variable compositions of IrO2 –SnO2 active
layers were prepared by thermal decomposition method.
Since a smooth surface with no oxides or scales is required
as the substrate for a uniform and well-adhered deposit, commercial Ti plates with a dimension of 10 mm × 10 mm
× 2.5 mm (i.e., an effective geometric area of 2.75 cm2 ),
were pretreated and used as the substrate. The substrates
were initially sandblasted. Subsequently chemical treatment
was carried out using 5% (w/w) oxalic acid solution for
10 min and 37% (w/w) HCl acid for 5 min, respectively.
Then, the substrates were dried at 100◦ C until they reach a
constant weight.
Two different precursor solutions were prepared by dissolving IrCl3 ·3H2 O in ethanol. The Ir metal concentrations
in the precursors were 0.1 and 0.3 M, respectively. 0.1 M
Ir coating solution was prepared by dissolving 0.4 g of
IrCl3 ·3H2 O in 6.18 ml ethanol. Similarly, 0.3 M Ir coating
solution was prepared by dissolving 0.8 g of IrCl3 ·3H2 O in
4.12 ml ethanol. 0.01g of polyethylene glycol (1000 ppm)
was added to both precursor solutions. The pretreated substrates were dip-coated in above solutions and the wet coating surfaces were air-dried in 80◦ C airflow to evaporate
solvents. Then, coating layers with different compositions
were calcinated in an oven at 450◦ C for 10 min. Electrodes
were washed with distilled water to remove the coated polymer. Subsequently, the second precursor solution, 2.5 mM of
Sn, was prepared by dissolving 2.43 ml of Sn (1000 ppm)
standard solution in 8.2 ml of isopropanol. Then, it was
applied on all the previously coated electrode surfaces. The
wet coating surface was then air-dried in 80◦ C air flow to
evaporate solvents and calcinated in an oven at 450◦ C for
10 min. The entire procedure was repeated till the final coating load of 1 mg cm−2 is achieved. After reaching the final
coating load, the electrodes were post-backed at 500◦ C in
a muffle furnace for 1 h. Our previous work indicated that
incorporating polyethylene glycol to precursor solution with
Ir makes pores on the layer and subsequent introduction of
precursor solution with Sn leads to better performances [17].
2.2 Microstructure analysis
Morphology of two electrodes was investigated using scanning electron microscope (SEM).
2.3 Electrochemical measurements
Electrochemical parameters related to anodic activity were
performed for two electrodes by cyclic voltammetry (CV)
in the range of voltage scan from –2.5 to 2.5 V at a 0.1
V s−1 scan rate. To assess the electrode stability of the
prepared anodes, open circuit potential (OCP) was performed. Anodic performance was measured by using anodic
polarization curves. The EIS measurements were conducted
covering the frequency range of 10 mHz–100 kHz in the
potential range between 1.2 and 1.35 V. The amplitude of
sinusoidal potential perturbation was 5 mV. The impedance
data were fitted to appropriate equivalent circuits using Nova
(1.10.19) software. All the tests were conducted using potentiostat/galvanostat equipment (Autolab, PGSTAT128N) in a
three electrode cell. The electrodes under study were used
as the working electrode (WE), titanium plate as the counter
electrode (CE) and Ag/AgCl electrode was used as the reference electrode (RE). In all the cases, Na2 SO4 (0.5 M) was
used as the electrolyte and experiments were carried out at
room temperature (25◦ C).
3.
Results and discussion
3.1 Microstructure
As shown in figure 1a and b, both anodes exhibit a typical
porous ‘cracked mud’ structure in some flat area. This morphology with dried-mud cracks is very classical for thermally
prepared DSAs and those cracked mud are oxide particles
[11]. It is observed that the coating morphology is affected
by the concentration of the precursor solution. As a matter
of fact, coatings obtained with diluted solution (0.1 M of Ir)
were less cracked while those obtained with a solution of 0.3
M Ir were more cracked.
3.2 Cyclic voltammetry and voltammetric charge analysis
Figure 2 shows the voltammetric curves of two Ti/IrO2 –SnO2
anodes in 0.5 mol dm−3 of Na2 SO4 at a scan rate of 0.1 V s−1 .
For these fresh anodes, well-defined peaks were found at 0.8
and 1.2 V. The broad peak observed at 0.8 V is attributed to
the surface redox transition of Ir(III)/Ir(IV), while the other
peak at 1.2 V is associated with the Ir(IV)/Ir(VI) transition
[18,19]. This means that the surface electrochemistry of the
electrode is governed by the active component of IrO2 . It is
in a good agreement with the production of an oxide layer
with a high degree of purity [19]. With the increase of Ir
concentration, the voltammetric curve shows similar shape,
but higher current density. Aromaa et al [20] also reported
that the current densities increase with the increasing
IrO2 concentration.
805
Oxygen evolution reaction of Ti/IrO2 –SnO2 electrode
Figure 1. Scanning electron microscopic images of electrodes. (a) Electrode coated
with (a) 0.1 and (b) 0.3 M Ir precursor solutions.
Table 1. Anodic voltammetric charge density (qa ) obtained from
the voltammograms of the Ti/IrO2 –SnO2 electrodes.
0.01
I (A cm –2)
0.008
0.006
Composition
0.004
0.1 M Ir
0.3 M Ir
0.002
Charge density/mC cm−2
67.93
102.80
0
– 0.002
–4
–2
0
2
4
– 0.004
– 0.006
– 0.008
– 0.01
E (Volts)
Figure 2. Cyclic voltammograms of (- -) 0.1 M Ir-coated electrode, (—) 0.3 M Ir-coated electrode. Scan range = 2.5 to –2.5 V,
scan rate = 0.1 V, electrolyte = 0.5 M of Na2 SO4 , reference electrode = Ag/AgCl. Arrow shows the direction of CV for 0.1 M
Ir-coated electrode.
The main electrochemical reactions which were occurred
in neutral Na2 SO4 solution are shown in equations (1) and
(2) [21].
−
IrO2 · 2H2 O + e− + Na+ → Na+ IrO (OH)2 H2 O , (1)
IrO2 · 2H2 O + e− + H+ → IrO (OH) · 2H2 O,
(2)
where the Na ion probably has a transient existence within
the reduced form of IrO2 , being replaced by H+ with time
and IrO2 is in its hydrous form.
The change in the anodic voltammetric charge density (qa )
obtained from the voltammograms are shown in table 1. The
voltammetric charge was determined by integration over the
whole potential region vs. Ag/AgCl. Voltammetric charge
density is an indication of the electrochemically active sites
present on the surface of the coating. It is seen that electrode
coated with 0.3 M Ir has higher charge compared to that of
the electrode coated with 0.1 M Ir. It can be speculated that
this difference in charge correlates to more defective/cracked
microstructure of electrode coated with 0.3 M Ir due to its
higher IrO2 content. Scanning electron micrographs further
confirmed the above speculation (figure 2). Therefore, it is
clear from these charges that more electrochemically active
sites are present and a large surface area is available for
these electrodes.
3.3 Polarization behaviour
OER on the Ti/IrO2 –SnO2 anode surface was studied
by polarization curves under quasi-steady-state conditions.
Results obtained for two electrodes are presented in figure 3.
Tafel coefficients obtained in the low and high overpotentials
are presented in table 2. Tafel lines show two slopes and
the slope becomes increasingly steep in the high overpotential region than in the low overpotential region. It can
be seen that the Tafel slope is 23.6 mV dec−1 for 0.1 M
coated electrode and 17.7 mV dec−1 for 0.3 M coated electrode in the low current density region. With increasing Ir
content, in the high current density region, this slope has
become increasingly steep from 158.9 mV dec−1 for 0.1 M
coated electrode to 116.6 mV dec−1 for 0.3 M coated electrode. This increment in the high current density region or
high overpotential region is usually attributed to the gas
bubble formation and pore clogging. This in turn may block
some electrochemically active surface area or change electrochemical reaction mechanism [3,22]. There are several proposed mechanisms which can describe the oxygen evolution
reaction in acidic media. The typical values of two slopes
in OER for IrO2 -based oxide electrodes are reported as
806
Gayani Chathurika Pathiraja et al
60 mV dec−1 in low overpotential region and 120 mV de−1
in high overpotential region in H2 SO4 solution [2,23]. However, Tafel slope values which were obtained in this research
are different from the reported literature in low overpotential
region. Since Tafel slopes indicate the electrocatalyst quality,
1.4
1.38
E/(V vs. Ag/AgCl)
1.36
1.34
1.32
1.3
1.28
1.26
1.24
1.22
1.2
1.18
0.003
0.004
0.005
0.006
Current/A
0.007
0.008
cm–2
Figure 3. Steady-state polarization curves performed in 0.5
mould m−3 of Na2 SO4 . (- - -) 0.1 M and (—) 0.3 M Ir-coated
electrodes. Scan rate = 0.1 mV s−1 .
Table 2. Tafel coefficients at low (b1 ) and high (b2 ) overpotentials Ti/IrO2 –SnO2 electrodes prepared by different Ir concentrations in 0.5 mould m−3 Na2 SO4 .
b1 (mV dec−1 )
b2 (mV dec−1 )
23.6
17.7
158.9
116.6
–Z″ (Ω)
0.1 M Ir
0.3 M Ir
3.4 Electrochemical impedance spectroscopy (EIS)
In the study of Ti/IrO2 –SnO2 as a porous electrode, it is
important to identify the main electroactive processes in
different domains of applied voltage. In addition, assessing the effects of the transport of electroactive species and
reactivity of the catalytic surface are important. EIS is a
good tool for obtaining such information, as EIS enables the
analysis of electrochemical structures and catalytic activities on the oxide matrix and their interface of the DSAs.
Figure 4a and b shows the EIS patterns of the Ti/IrO2 –
SnO2 electrodes with two different IrO2 concentrations under
2.5
2.5
2
2
1.5
1.5
–Z″ (Ω)
Composition
lower value of Tafel slope represents faster kinetics of
the reaction [24]. Moreover, agglomerates can be generally
found that IrO2 -coated titanium anodes derived from conventional thermal decomposition and has been reported in
the literature [25]. In the case of our anode, the electrocatalysts may consist of agglomerates which may have different
reaction sites. Therefore, different rate-determining steps for
different reaction sites can coexist and the measured Tafel
slope may represent an average. Therefore, it is unclear in
this case which mechanism is operating on these electrodes
and further research in this context can be interesting.
The Tafel slope of 0.3 M coated electrode is 116.61 mV
dec−1 , which is close to 2.303 (2RT/F). In addition, it has
been reported that OER on the thermal IrO2 in neutral
solution is difficult [3,26]. Similar behaviour of OER with
two Tafel slopes has been reported for IrO2 –Ta2 O5 electrodes in 0.5 mould m−3 of Na2 SO4 [3]. The other value
of the Tafel slope in high overpotential region (158.9 mV
dec−1 ) is also not complying with the reported standard
values. This suggests that the mechanism is presumably a
composite one. Additionally, since change in Tafel slope
represents a difference in electrochemical reaction mechanism, it can be suggested that these two electrodes have
different mechanisms.
1
0.5
0
(a)
1
0.5
2
4
6
Z′ (Ω)
8
0
10
(b)
2
4
6
8
10
Z′ (Ω)
Figure 4. Nyquist plots of Ti/IrO2 –SnO2 electrodes in 0.5 mould m−3 Na2 SO4 at different
potentials: (a) 0.1 M and (b) 0.3 M Ir-coated electrodes. () 1.25 V and () 1.3 V vs. Ag/AgCl
electrode.
807
25
25
20
20
Phase angle/deg
Phase angle/deg
Oxygen evolution reaction of Ti/IrO2 –SnO2 electrode
15
10
15
10
5
5
0
–2
0
(a)
2
0
–2
4
(b)
log(f/Hz)
0
2
4
log(f/Hz)
Figure 5. Bode plots of of Ti/IrO2 –SnO2 electrodes in 0.5 mould m−3 Na2 SO4 at different
potentials (a) 0.1 M and (b) 0.3 M Ir-coated electrodes. () 1.25 V and () 1.3 V vs. Ag/AgCl
electrode.
Qdl
L
Rs
Q2
The inductance (L) is resulted from the wiring and measuring equipment components. Rs is the solution resistance
between the reference and working electrodes, while Rct is
the charge transfer resistance for oxygen evolution reaction.
The pseudocapacitance used in the model is represented by
a constant phase element (CPE) and its impedance is represented by the expression given in equation (3) [28,29]:
Rct
R2
Figure 6. Equivalent circuit model for EIS analysis of Ti/IrO2 –
SnO2 electrodes.
oxygen evolution reaction at different anodic potentials in
0.5 mould m−3 Na2 SO4 solution. It can be observed in both
anodes (separately) that the complex plane in the whole frequency, phenomenally domain, shows only one capacitance
arc, decreasing with the electrode potential. Moreover, as can
be clearly seen, the capacitive region shifts to higher frequencies as the IrO2 concentration decreases, indicating a lower
capacitance value.
The fit results of Bode plots with the above equivalent circuit are also shown in figure 5a and b. The deviation between
the experimental data and the simulation result is small, suggesting that the used equivalent circuit is suitable to represent
the electrochemical system. The fitting of equivalent electrical
circuits (EEC) to the EIS results were performed to establish the possible differences between the physical and chemical characteristics of both electrodes when in contact with a
liquid phase. The equivalent circuit [LRs (Qdl [Rct (Q2 R2 )])],
shown in figure 6, has been used to fit the EIS data using
Nova 1.10. Similar impedance parameters were obtained
from fitting the experimental data to the Rs (R2 Q2 )(Rct Qd1 )
circuit, for platinum activated IrO2 /SnO2 nanocatalysts [27].
ZCPE =
1
,
Q (iω)n
(3)
where ω is the angular frequency, Q is a frequency independent constant and n is a power factor related to the depression
angle. n is in the range varying from 0 to 1.0 and directly
associated with the dispersive character of the system. An n
value of zero corresponds to a pure resistor.
Here the constant phase angle elements Q2 and Qdl are
used to replace the film capacitance and double layer capacitance for a better simulation of the spectra due to the
depression effect, which is attributed to the roughness and
inhomogeneity of the electrode surface. R2 and Q2 are resistances and pseudocapacitances associated with two solidstate surface redox transitions, which are possibly due to
electrochemical porosity occur at the electrode material. The
use of pseudo-capacitances instead of pure capacitors is due
to the heterogeneous nature of the surface. Both CPE parameters depend on the electrode potential and when n values are
close to 1, the value of ZCPE approaches the impedance value
of a conventional capacitor.
EEC parameters obtained through fitting are listed in
table 3. It is not exactly clear the source of the small inductive
element, L, possibly resulting from testing wiring, electrical connections and measuring equipment components. The
value of this electronic originated inductance was reported
as about 1 μH, which is in good agreement with our fitting result [1,11]. Da Silva et al [30] have suggested that the
main cause of the inductance can be due to the properties
808
Gayani Chathurika Pathiraja et al
Table 3. The fitting EIS parameters of Ti/IrO2 –SnO2 electrodes
prepared by different Ir concentrations at 1.3 V.
80
70
Qdl
(μ Mho)
Rct
()
n
0.1 M Ir-coated anode
0.3 M Ir-coated anode
212
182
17.4
16.9
2.65
2.68
14.0
10.7
0.895
0.940
of porous nature of the thermally prepared metallic oxide
electrode materials.
The change of charge-transfer resistance (Rct ) for OER
with the composition of anodes was measured at 1.3 V potential, as shown in table 3. Rct was slightly decreased from
14.0 to 10.7 cm−2 with increasing IrO2 content, showing
the slight enhancement of the catalytic activity of 0.3 M Ircoated anode. Conversely, the reciprocal of Rct represents the
electrochemical activity of OER on the surface of the anode.
The Rct for 0.1 M coated anode is 0.093 cm−2 , which is
much higher than the other anode (0.071 cm−2 ). This indicates its poor electrochemical activity. This is consistent with
the results of polarization measurement. The film resistance,
Rs , is originated from both the mixed metal oxide coating and
the interlayer between the oxide coating and the substrate.
The change of the oxide film resistance with two compositions of Ti/IrO2 –SnO2 anodes show that the film resistance
dropped slightly from 17.4 to 16.9 , when coated Ir content
increased from 0.1 to 0.3 M.
As shown in table 3, n values of both anodes are close
to 0.9–1, which suggests that the CPE for oxygen evolution on Ti/IrO2 –SnO2 anodes has the nature of a capacitor.
Moreover, the Qdl can be taken approximately as the double layer capacitance Cdl , which is also related to the electrochemically active surface area of the oxide coating [14].
However, the values of Cdl of both anodes at that potential
are almost similar and do not reflect the increment of electrochemically active surface area. This means inner surface
makes a large contribution to the electrochemically active
surface area of Ti/IrO2 –SnO2 anodes. However, the cyclic
voltammetry study showed that 0.3 M Ir-coated anode has
more IrO2 agglomerates and hence a large electrochemically
active surface area. This may be due to the fine crystallites
which produce more grain boundaries to enlarge the inner
surface area.
The change in double layer capacitance, Cdl , as a function of the applied potential is shown in figure 7. At 1.2 V,
Cdl shows its maximum and then it suddenly drops with
increasing potential. It is interesting to observe that its maximum coincides with the peak potential, corresponding to
the Ir(IV)/Ir(VI) solid-state redox transitions, observed in the
cyclic voltammograms of both anodes. Moreover, it is clearly
seen that Cdl depends on potential as well as composition.
The increment of IrO2 content in the 0.3 M coated anode
resulted in an increase in the Cdl value at 1.2 V. Lassali et al
[14] also observed that the higher Cdl value at 1.2 V and a
maximum at around 0.9–1.0 V/RHE, corresponding to the
Ir(III)/Ir(IV) solid-state redox transitions.
60
Qdl (mF cm –2)
Rs
()
50
40
30
20
10
0
1.15
1.2
1.25
1.3
1.35
1.4
E/(V vs. Ag/AgCl)
Figure 7. Double layer capacitance Cdl as a function of the potential of Ti/IrO2 –SnO2 electrodes. () 0.1 M and (♦) 0.3 M Ir-coated
electrodes.
0
– 0.1
– 0.2
E/(V vs. Ag/AgCl)
Anode
L
(μH)
– 0.3
– 0.4
– 0.5
– 0.6
– 0.7
– 0.8
– 0.9
–1
0
200
400
Time (s)
600
800
Figure 8. Open circuit potential of different Ti/IrO2 –SnO2
electrodes. (- - -) 0.1 M and (—) 0.3 M Ir-coated electrodes.
3.5 Open circuit potential (OCP)
From a practical point of view, electrode stability is always
the main concern. Open circuit potential (OCP) measurements were carried out to assess the stability of the anodes
under investigation. Figure 8 shows the OCP curves for both
anodes immersed in 0.5 M of Na2 SO4 solution at 25◦ C. The
0.3 M coated electrode has –0.74 V OCP value, while 0.1 M
coated electrode has –0.82 V OCP value. OCP values of
both anodes is negative, since surface of IrO2 is negatively
charged. The reason is due to the zero charge of IrO2 is
less than 1 in Na2 SO4 solution. Alves et al [15] observed
Oxygen evolution reaction of Ti/IrO2 –SnO2 electrode
similar negative values and these values were also dependent
on the composition. Moreover, 0.3 M coated anode stabilizes quickly around a higher stationary value than 0.1 M
coated anode. This stable potential occurs with the oxidation
of the surface of electrode. OCP measurements performed
were also in agreement with other results.
4.
Conclusions
The surface electrochemistry of Ti/IrO2 –SnO2 anodes are
governed by the Ir(III)/Ir(IV) couple. Ti/IrO2 –SnO2 anode
with 0.3 M Ir has both the largest electrochemically active
surface area and electrochemical activity. This is due to the
more defective/cracked microstructure of electrode coated
with 0.3 M Ir, due to its higher IrO2 content.
The Tafel slope of 0.3 M coated electrode is 116.61 mV
dec−1 , close to 2.303 (2RT/F) in high overpotential region,
while other electrode has Tafel slope of 158.9 mV dec−1 .
Therefore, it is unclear in this case which mechanism is
operating on these electrodes.
The equivalent circuit that matches our experimental data in terms of impedance measurements is
[LRs (Qdl [Rct (Q2 R2 )])]. Rct was slightly decreased from
14.0 to 10.7 cm−2 with increasing IrO2 content, showing
the slight enhancement of the catalytic activity of 0.3 M
Ir-coated anode. In the range of potentials investigated here,
both CPEs for oxygen evolution on Ti/IrO2 –SnO2 anodes
have the nature of a capacitor. However, the values of Cdl
of both anodes at 1.3 V potential are almost similar and do
not reflect the increment of electrochemically active surface area. This shows that the inner surface makes a large
contribution to the electrochemically active surface area of
Ti/IrO2 –SnO2 anodes.
It can be seen that OCP of 0.3 M coated electrode is −0.74
V, while 0.1 M coated electrode is −0.82 V. This analysis
also permits to conclude that changes in the IrO2 composition
of the electrode during OER, affect the stability.
Acknowledgement
This work was supported by the National Research council
(NRC), Sri Lanka (grant no: 11-054).
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